Valentines day isn't just for redrom, you guys. It's a time to celebrate your kismesis and matesprit both equally.
I figure the finger-across-the-throat/throat slit gesture is like the blackrom equivalent of blowing a kiss. That's what Eridan's doing in that last panel, I couldn't figure out how to make that clearer. Also I forgot to draw in a journal or something in the first panel so I guess Sollux is just drawing on the desk or something. OH WELL wwhatevver.
(Oh Eridan, you are such a massive fuckup.)